1. Field of the Invention
The present invention relates to an auto tensioner such as that used to maintain an appropriate tension in various types of belts such as a timing belt of an automotive engine.
2. Description of the Prior Art
Mechanisms involving a belt drive are widely used for driving the cam shaft of an OHC, or DOHC engine synchronously with the crank shaft. With these mechanisms an appropriate tension is obtained in the belt by resiliently urging a tension pulley against the belt.
Conventional well-known auto-tensioners such as the one disclosed in Japanese Patent First Publication KOKAI No.63-180759 utilize the resilient force of a spring to urge the tension pulley resiliently against the belt. A typical example of the construction of such an auto-tensioner is shown in FIGS. 11 and 12.
With this construction, a fixed shaft 1 having a central axis "a" is fixedly attached for example to the front face of an engine block. A pivotal member 2 having a front or forward portion 4 (left side portion in FIG. 12) and a rear or root portion 7 (right side portion in FIG. 12) is supported on the peripheral face of the fixed shaft 1 by means of sliding bearings 3a and 3b so as to be freely pivotal thereabout. The front portion 4 which has a central axis "b" displaced with respect to the central axis "a" of the fixed shaft 1, supports a tension pulley 5 on an outer peripheral face thereof by means of a rolling bearing 6 so as to be freely rotatable thereabout. A bracket 8 having a rim in which is formed an engaging member 9, is fixedly disposed on the peripheral face of the rear portion 7 of the pivotal member 2. By engaging the end of a tension spring (not shown in the figure) to the engaging member 9, a resilient force can be applied to the pivotal member 2 in a direction of rotation of the pivotal member 2 about the fixed shaft 1. As a result, the outer peripheral face of the tension pulley 5 can be resiliently urged towards the belt.
A multi-plate damper mechanism is provided between the fixed shaft 1 and the pivotal member 2 so that even with small fluctuations of the belt, these movements are not followed immediately by the tension pulley 5. Hence vibration of the belt can be effectively damped. The multi-plate damper mechanism comprises a plurality of fixed side plates 11 and movable side plates 12 arranged alternately within a space 10 provided between the outer periphery of the fixed shaft 1 and the inner periphery of the pivotal member 2 and closed by seals 17 and 18 located at axially opposite ends thereof. The inner peripheral rims of the fixed plates 11 are supported on the fixed shaft 1 so as to be rotationally fixed relative thereto, while the outer peripheral rims of the movable plates 12 are supported on the inner peripheral face of the pivotal member 2 so as to be rotationally fixed relative thereto. A retaining plate 30 is fixedly disposed on a peripheral portion of the fixed shaft 1 so as to maintain the spacing between the fixed side plates 11 and the movable side plate 12. The space 10 accommodating the fixed and movable side plates 11 and 12 is filled with a viscous liquid such as oil through a filling hole 13, which is closed after filling by means of a ball 4.
The side faces of the multiple fixed and movable side plates 11 and 12 are separated by a very small gap which is filled by the viscous fluid. Hence any sudden relative fluctuations between the plates 11 and 12 are damped by the resultant shear forces in the viscous fluid so that even with a local sudden increase in belt tension this is not immediately followed by the tension pulley 5. Accordingly, the tension pulley 5 is held against the belt even when the belt is vibrating, thereby damping growth of the vibration.
With the pivotal member 2 of the conventional auto tensioner, the outer diametric dimension of the front portion 4 must be accurately finished to provide a press fit into the inner raceway 15 of the bearing 6, and the outer diametric dimension of the rear portion 7 must be accurately finished to provide a press fit into the bracket 8. Consequently, when manufacturing the pivotal member 2, a blank 16 having outer diametric dimensions larger than those of the finished pivotal member 2, as shown by the chain line in FIG. 13 is produced by forging. The hatched portions shown in FIG. 13 are then removed by machining to give an accurate finish to the outer diametric dimensions of the front portion 4 and the rear portion 7 in the pivotal member 2.
As well as proving difficult and time consuming, the machining process for making the front and rear portions 4 and 7 eccentric with respect to each other involves intermittent cutting which results in excessive abrasion of the cutting tool used. Thus manufacturing cost for the pivotal member 2 are high so that the auto tensioner incorporating the pivotal member 2 is also expensive.